Lead gallery bullets are well known; they are characterized by the use of powders of lead consolidated into a bullet having sufficient strength for use and intended to be disrupted into small fragments on impact with a gallery target.
The costs associated with the training of users of such ammunitions are extremely high. First, in a shooting gallery, an expensive device, called "bullet trap", is required to stop the projectile to prevent fragments from injuring shooters. Furthermore, the walls of the shooting galleries must be covered with "ballistic rubber" in order to stop occasional fragments of the projectile.
There is also lead contamination which bears a heavy burden in the cost associated with training with standard ammunitions. During a shooting session, there is an emission of lead dust into the atmosphere. Also, the accumulation of projectiles in shooting galleries causes an environmental problem. Many shooting galleries have been closed recently due to the high level of lead in these installations.
The problem is replacing lead for the purpose of a gallery shooting round is to find a material sufficiently heavy that the automatic weapons will be able to cycle and the shooter will see few differences. Costly or toxic heavy metals should be avoided while a cheap production process is required to keep production costs low.
The main criteria for the ability of a round to cycle autoloader weapons is the amount of energy that it delivers to the cycling mechanism. For some type of weapons, this energy is delivered by the expanding gases pushing back the cartridge case. This type may be found with the 9 mm Browning Hi-Power pistol for example. For some others, high pressure gases are connected through a port pressure inside the barrel. The high pressured gases are then the source of energy for the cycling mechanism. This type is found in most 5.56 NATO nominated weapons, like the Colt M16.
Weapons and propellant powders are designed to work with a projectile of a certain mass that gives a typical pressure-vs-time curve. Using a lighter projectile will cause problems, the main one being too low an energy transfer to give the feeding mechanism the needed momentum to cycle, in certain type of weapons.
In order to replace lead in a projectile, the selected material should have a minimum specific gravity so that the resulting projectile mass is compatible with commercially available propellants for that calibre. This is important since it would not be economically viable to develop a lead-free round where a special propellant or other component would need to be developed.
It has been found that, with 5.56 mm autoloader weapons, the minimum specific gravity for a lead replacement material that would allow reliable cycling of most of these weapons would be 5.7. This specific gravity makes it possible to reach the same port pressure as with standard rounds, using the same propellants, but with a higher charge.
Training bullets including plastics material, either encapsulating or filled with metal powders, have been proposed to meet this problem.
European patent number 0,096,617, issued to Societe Francaise de Munitions, describes a training bullet having a mixture of nylon, a powder of a ductile metal and a solid lubricant. This patent describes practice ammunitions wherein the specific gravity of the compound is between 3 and 5.
International patent application PCT 88-09476 describes a bullet comprising a matrix of plastics material having a water absorption factor similar to or greater than that of nylon 66 containing a filler material effective to raise the specific gravity of the bullet from 3 to 7. However, when copper is used, its content by weight is limited to 88%; whenever a higher percentage of metal powder is desired, copper must be mixed with another metal filler, such as tungsten (46.5% by weight).